The present invention relates to a refrigerant flow control device for used with a refrigerator cycle such as air conditioner and in particular to a refrigerant flow control device suitable to flow control when the liquid back state has occurred in the evaporator.
A conventional refrigerant flow control device in which the opening of the expansion value can be adjusted by an electrical signal will be first explained.
In order to evaporate the refrigerant in the evaporator moderately according to the external thermal load in the conventional refrigeration cycle, it is well known to provide temperature sensors at the inlet and the outlet of the evaporator, or to provide a temperature sensor and a pressure sensor at the outlet of the evaporator as described in U.S. Pat. No. 4,505,125 issued Mar. 19, 1985 in the name of Richard A. Baglione, and to obtain, on the basis of signals supplied from those sensors, the degree of super-heat of the refrigerant at the outlet of the evaporator and to control, by a refrigerant flow control device, the opening of the expansion valve thereby controlling the flow rate of the refrigerant in the refrigeration cycle, as to make the degree of super-heat a predetermined value.
In such a conventional refrigerant flow control device, the super-heat degree SH of the refrigerant measured at the outlet of the evaporator becomes zero and the deviation e of the measured super-heat degree SH from its desired value SH.sub.o is represented as e=-SH.sub.o when the refrigerant at the outlet of the evaporator assumes the two-phase state due to the decrease in the air flow rate of the cooling fan of the evaporator or the decrease in the thermal load within the room to be cooled. Even if the thermal load is then small or comparatively large near the saturated vapor line, the deviation signal e always assumes the constant value SH.sub.o. Therefore, if the super-heat degree is decreased progressively to reach zero due to decrease in thermal load, for example, as illustrated in FIGS. 9 and 10, the deviation signal e assumes the constant value as represented by e.sub.o =-SH.sub.o irrespective of the status position of the outlet of the evaporator. Thus, the opening of the expansion valve is disadvantageously adjusted based on the value of e.sub.o to control the refrigerant flow rate. Thus, even if the refrigerant flow rate should be rapidly decreased, the opening of the expansion valve would change slowly because of small absolute value of the deviation signal e. Accordingly, the refrigerant at the outlet of the evaporator will assume the two-phase state, namely so-called liquid back state for a long time, resulting in a drawback of shortened life or breakage of the compressor. Further, in a device for controlling the super-heat degree of the refrigerant at the outlet of the evaporator, the deviation e is always constant irrespective of the control signal supplied to the expansion valve. This results in a drawback of unstable control.